Dear LF group,
In Rik's formulae, if you make W in m equal d in mm, the formulae reduce to
L=1.1W, give or take a few percent,depending on shape - close to RA9MB's
formula L=W for a single turn. I have not tried a big TX loop, but the 2m
side square loops made of 15mm diameter tube I have been using lately have
L=7.9uH, very close to Rik's square loop formula and Yuri's simple formula;
a 1m square loop of the same tube had L about 4uH. Making the wire thinner
increases L, but is not very critical because of the Log component. A 1m
square loop of about 1mm wire had L of about 6.4uH, which agrees with Rik's
formula which predicts 6.1uH.
Alex's formula L=WN^2 makes the inductance proportional to N^2 for a
multi-turn loop; in practice the inductance will always be somewhat less
because there is not unity coupling factor between the turns. A 10 turn,
1m^2 loop made using 1mm PVC insulated wire in a bundle had an L of about
340uH rather than 400uH The error will depend on the spacing of the turns
and how they are bundled together and so on. All the same, it seems like a
good way of making a rough estimate - in practice, there will be many
factors that have a slight effect on the actual value of L, so a rough
estimate is probably all that is required.
Cheers, Jim Moritz
73 de M0BMU
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